docs: def _mergex(xlo, xhi):
      """
      We are assuming that points are consecutive, so that xlo[n]=xhi[n-1]
      This is usually valid for fits from a single spectrum, but breaks for
      simultaneous multiwavelength fitting
      """
      N = xlo.size
      x = np.zeros(N + 1)
      x[:N] = xlo.copy()
      x[-1] = xhi[-1]
  
      return x